1. Field of the Disclosure
The present disclosure relates to a blade tip for a rotary blade e.g. a rotary blade for a gas turbine engine, and to a method of manufacturing a metal foam article such as a blade tip for a rotary blade.
2. Description of the Related Art
It is desirable to reduce the clearance between the tip of a rotary fan blade and the fan casing of a gas turbine engine in order to maximise fuel efficiency. Gas (air) leaking through the clearance gap rather than passing through the blades of the fan represents lost energy.
However, reducing the clearance between the fan blade tip and the fan casing can lead to undesirable rubbing of the fan blade tips on the fan casing during running engine conditions. The rubbing can cause blade damage both at the tip (resul0ting from the direct contact with the casing) and throughout the blade structure (resulting from stresses arising from vibrations and thermal damage arising from frictional heat caused by the rubbing). Furthermore abraded material can block cooling holes in the blade.
Various ways of reducing rubbing between the blade tips and fan casing during running conditions are known. For example, it is known to increase the clearance between the blade tip in their cold/static state and the fan casing to allow for dimensional changes in the blade tips during running. However, this reduces the turbine efficiency especially in the early stages of engine running.
It is known, for example from EP0659978A to effectively reduce the clearance between the blade tips and the fan casing during engine running by injecting internal cooling air into the gap between the blade tip and the fan casing or by channelling gas (air) from the pressure face of the blade through a curved passageway to the tip of the blade to create a buffer zone to prevent gas (air) from leaking through the clearance gap. The curved passageways are formed by drilling methods such as electrochemical milling or laser beam drilling.
Another known way for attempting to reduce rubbing under normal rubbing conditions is to provide a track liner on the interior surface of the fan casing to provide a shroud for the fan blade tips. The track liner may be formed of an abradable material (e.g. an epoxy resin) which is abraded by the rotary fan blades to form channels in the track liner into which the fan blade tips extend. These abradable track liners have found some success with metallic fan blades (e.g. titanium blades) which are very durable but a disadvantage of these track liners is that the running clearance is set by the longest blade. These track liners are not appropriate for use with fan blades formed of low temperature composite material (e.g. fibre-reinforced plastic material) which are less durable and can suffer blade tip damage.
Ablative fan casings are also known where the tip sections of the rotary fan blade are formed of an abradable material and are abraded by the fan casing during rubbing. Again, damage to the blade structure can occur as a result of uncontrolled vibrational and thermal energy arising during rubbing.